In a typical electrical distribution system, electrical energy is generated by an electrical supplier or utility company and distributed to consumers via a power distribution network. The power distribution network is the network of electrical distribution wires which link the electrical supplier to its consumers. Typically, electricity from a utility is fed from a primary substation over a distribution cable to several local substations. At the substations, the supply is transformed by distribution transformers from a relatively high voltage on the distributor cable to a lower voltage at which it is supplied to the end consumer. From the substations, the power is provided to end consumers, such as industrial users, over a distributed power network that supplies power to various loads. Such loads may include, for example, various power machines or computer/electronic equipment.
At the consumer's facility, there will typically be an intelligent electronic device (“IED”), such as an electrical energy/watt-hour meter, connected between the consumer and the power distribution network so as to measure quantities such as the consumer's electrical consumption or electrical demand. Such a meter may be owned by the consumer and used to monitor and control consumption and report costs or may be owned by the utility and used to monitor consumption and report revenue.
An electrical power meter is generally an intelligent electronic device that records and measures electrical power consumption. Electrical power meters include, but are not limited to, electric watt-hour meters. In addition, electrical power meters are also capable of measuring and recording power events, such as brown outs, spikes, sags or swells, power quality, current, voltages waveforms, harmonics, transients or other power disturbances. Revenue accurate meters (“revenue meter”) are revenue accuracy electrical power metering devices which may include the ability to detect, monitor, or report, quantify and communicate power quality information about the power which they are metering.
The electrical power meter is a critical part of the electric utility infrastructure. Meters keep track of the amount of electricity transferred at a specific location in the power system, most often at the point of service to a customer. Like the cash-register in a store, these customer meters are the place where the transaction occurs, where the consumer takes possession of the commodity, and where the basis for the bill is determined. Unlike a cash-register, however, the meter sits unguarded at the consumer's home and must be trusted, by both the utility and the home owner, to accurately and reliably measure and record the energy transaction.
Electricity is not like other commodities because it is consumed in real-time. There is nothing to compare or measure later, nothing to return, nothing tangible to show what was purchased. This makes the meter all the more critical for both the utility and the consumer. For this reason, electrical power meters and the sockets into which they are installed are designed to standards and codes that discourage tampering and provide means of detecting when it is attempted. Intentional abuses aside, the electrical power meter itself must be both accurate and dependable, maintaining its performance in spite of environmental and electrical stresses.
In more recent years the electric utility marketplace has moved towards deregulation where utility consumers will be able to choose electrical service providers. Until now, substantially all end users purchased electric power they needed from the local utility serving their geographic area. Further, there was no way for utilities to guarantee the same reliability to all consumers from the utility because of different connection points to the transmission and distribution lines. With deregulation it is essential for consumers to be able to measure and quantify power consumption and reliability from their suppliers in order to ensure they are receiving the service they have opted for. Such service may involve various pricing plans, for example on volume, term commitments, peak and off-peak usage or reliability.
Electrical power meter accuracy is typically measured as a function of the percentage of error in measurement over a particular measurement range. The accuracy rating of an electrical power meter is governed by defined industry and regulatory standards including American National Standards Institute (ANSI) C12.20 for North America and IEC 62053 for areas outside North America. The C12.20 standard establishes the physical aspects and performance criteria for a meter's accuracy class. Accuracy classes are defined and used in IEC and ANSI standards. Classes are denoted by either a letter or percentage. For example, Class B is a temperature accuracy from IEC-751 that requires accuracy of +/−0.15 degrees Celsius. Class 0.5 is an ANSI C12.20 accuracy class for electric meters with accuracy of +/−0.5%. Typically, accuracy is measured against a nominal (maximum) rated value and may vary at lower values.
Keeping in-step with the technology improvements associated with solid state metering, ANSI developed new standards with more stringent accuracy requirements during the late 1990's. ANSI C12.20 established Accuracy Classes 0.2 and 0.5, with the Class numbers representing the maximum percent metering error at normal loads. Typical residential solid state electricity meters are of Class 0.5, whereas electro-mechanical meters were typically built to the less stringent ANSI C12.1. In addition, C12.20 compliant meters are required to continue to meter down to 0.1 A (24 Watts), whereas C12.1 allowed metering to stop below 0.3 A (72 Watts). While metering of such low loads is not likely significant on a residential bill, it is an accuracy improvement nonetheless. The existing ANSI accuracy classes for electric meters are:
Class 0.5—having ±0.5% accuracy; and
Class 0.2—having ±0.2% accuracy.
Manufacturers and utilities use a range of tests and equipment to verify that meters adhere to the ANSI and IEC requirements. During the manufacturing process, it is common that each individual meter is calibrated and verified. Once a utility receives new meters, there is often another accuracy test, either on each meter or on a sample basis. States generally establish requirements for how utilities are to check accuracy when new meters are received and at intervals thereafter.
Various factors affect the accuracy of an electrical power meter, including environmental factors, such as temperature and/or humidity of the operating environment, internal factors, such as the quality and tolerances of the meter's components, and other factors such as the quality of the electrical power being monitored. To account for these various factors which affect accuracy, an electrical power meter typically undergoes a calibration process, performed by the manufacturer or by the consumer, by which the measurement mechanisms are tested under controlled conditions, any inaccuracies are determined and the adjustments are made so as to account for the measured inaccuracies during actual operation. Calibration processes may include calibration for gain errors, such as errors caused by component tolerances, offsets, such as offsets internally added to measured signals for internal uses, phase errors, such as errors caused by mismatched delay among measurement channels, drift, such as errors caused by components falling out of tolerance over time, and noise, such as noise injected into signals by the meter's components. Such calibration processes may be time consuming, thereby delaying manufacturing, and typically require highly accurate and costly external signal sources and measurement equipment.